Yankee dryers have been used for many years to dry wet paper webs during the paper-making process. Typically, the wet paper web is delivered to and pressed into engagement with the rotating dryer drum with the outer cylindrical surface of the dryer drum delivering the wet paper web to a dryer hood associated with the dryer drum. The dryer drum is heated internally by steam or the like so that the outer surface thereof is maintained at a high temperature. After entering the dryer hood the wet paper web is also subjected to heat on the outer surface (air side) thereof.
It is conventional practice to employ a plurality of nozzle boxes in yankee dryers, the nozzle boxes normally being arrayed in a cross-machine direction to direct heated air against the top surface of the web being dried along the width thereof. Typically, the nozzle boxes are supported by crescent headers arrayed in the machine direction as well as by curved support side plates at the sides of the hood.
Conventional hoods are constructed in such a way that the entire internal structure grows with temperature. In conventional arrangements, the support side plates are directly exposed to the very hot temperatures within the hood compartments. Heating of the support side plates will cause them to deform and tend toward straightening of the plates. The crescent headers are subject to the same action. This deformation results in a lack of concentricity, moving at least some of the nozzle boxes (particularly the end-most boxes) away from the dryer drum and causes operating inefficiencies. Convection heat transfer drops off dramatically as the nozzle boxes move away from the sheet.
It will also be appreciated that extremely high temperatures and variations in temperature can cause potentially harmful stresses to build up in the nozzle boxes and related components, resulting in both structural and performance degradation unless the problem of expansion and contraction due to high temperatures and temperature changes is addressed and corrected.
Another difficulty arising in prior art hood constructions is the expansion of the nozzle boxes which results in significant movement at one end thereof, the other nozzle box end being fixed in position on a structural side plate. This causes misalignment between the nozzle box and a sheet edge.